1. Field of the Invention
The present invention relates to a drawing method for drawing a pattern on a surface of a workpiece, using an exposure unit including a plurality of optical modulation elements arranged in a matrix manner, and also relates to a drawing apparatus in which the drawing method is executed.
2. Description of the Related Art
In general, such a drawing apparatus is used for optically drawing fine patterns, characters, symbols and so on the surface of a suitable workpiece. A representative use of the drawing apparatus is for a circuit pattern drawn on a workpiece when a printed circuit board is manufactured by using photolithography. In this case, the workpiece may be either a photosensitive film for producing a photomask or a photoresist layer formed on a suitable substrate.
Recently, circuit pattern drawing systems have been developed for efficiently carrying out the drawing of circuit patterns by integrating the use of a drawing apparatus, a CAD (Computer Aided Design) station for designing a circuit pattern, and a CAM (Computer Aided Manufacturing) station for editing the designed circuit. The drawing apparatus, the CAD station, and the CAM station are all connected to each other through a LAN (local area network).
A circuit pattern is designed and treated as vector-graphic data by the CAD station. The vector-graphic data may be fed from the CAD station to the CAM station, through the LAN, for editing the designed circuit pattern, if necessary. The vector-graphic data is fed from either the CAD station or the CAM station to the drawing apparatus, and is stored in a storage medium, such as a hard disk, provided in the drawing apparatus.
The drawing apparatus is provided with a vector-to-raster converter, and a bit-map memory. The vector graphic data is read from the hard disk, for example, and is converted into raster-graphic data by the vector-to-raster converter. Then, the raster-graphic data are developed and stored in the bit-map memory for a drawing operation.
The drawing apparatus is further provided with an exposure unit for drawing a circuit pattern on the workpiece based on the raster-graphic data, and the exposure unit may comprise a digital micro-mirror device (DMD) unit, a liquid crystal display (LCD) array unit or the like.
As is well known, the DMD unit has a reflecting surface which is formed from a plurality of micro-mirror elements arranged in a matrix manner. Each micro-mirror element is independently driven between a first reflecting position and a second reflecting position. Thus, it is possible to divide a light beam, which is wholly made incident on the reflecting surface of the DMD unit, into a plurality of light beams by independently driving each of the micro-mirror elements between the first and second reflecting positions.
In the DMD unit assembled in the drawing apparatus, when each micro-mirror element is at the first reflecting position, it reflects the light beam such that the reflected light beam is directed toward the workpiece, and when each micro-mirror element is at the second reflecting position, it reflects the light beam such that the reflected light beam deviates from the workpiece. Thus, since each of the micro-mirror elements serves as an optical modulation element, it is possible to draw the circuit pattern on the workpiece by operating the DMD unit in accordance with the raster-graphic data read from the bit-map memory.
Also, as is well known, the LCD array unit comprises a pair of transparent plate members with a liquid crystal intervened therebetween, and plural pairs of transparent electrode elements arranged on and attached to the transparent plate members in a matrix manner so that the transparent electrode elements in each pair are registered with each other. With the arrangement of the LCD array unit, it is possible to selectively allow penetration of a light beam through each pair of transparent electrode elements by controlling application of a voltage to the electrode elements in pairs. Thus, since each pair of the transparent electrode elements also serves as an optical modulation element, the circuit pattern can be drawn on the workpiece by operating the LCD array unit in accordance with the raster-graphic data read from the bit-map memory.
Furthermore, the drawing apparatus is provided with a light source device for generating and introducing a light beam into the exposure unit. A suitable lamp, such as a light emitting diode (LED), a high-pressure mercury-vapor lamp, a xenon lamp, a flash lamp and so on, is used in the light source device, according to the sensitivity and the optical characteristics of the photoresist layer on the workpiece.
Usually, a circuit pattern to be drawn on the workpiece has an area which is considerably larger than an exposure area obtained from an exposure unit or from several exposure units. Thus, it is necessary to scan the workpiece with at least one exposure unit before a large circuit pattern can be completely drawn on the workpiece. To this end, the drawing apparatus is provided with a movable drawing table on which the workpiece is placed. The drawing table is moved relative to the exposure unit, whereby the workpiece on the drawing table is scanned with the exposure unit.
Conventionally, a “step & repeat” method is used as a scanning method in the drawing apparatus. In particular, in the “step & repeat” method, the drawing table is intermittently moved relative to the exposure unit, so that a part of the circuit pattern is drawn on the workpiece by operating the exposure unit based on raster-graphic data during the stoppage of the drawing table, and the intermittent movement of the drawing table is continued until the circuit pattern is completely drawn.
There are various drawbacks in the “step & repeat” method. First, the drawing speed is slow because of the intermittent movement of the drawing table. Namely, the drawing is not performed during the movement of the drawing table, and therefore the entire drawing time must be calculated as the sum of the movement time of the drawing table and the stoppage time of the drawing table. It is possible to increase the drawing speed, using a high-power light source device, but the high-power light source device is costly.
Also, the drive mechanism for driving the drawing table is susceptible to damage because acceleration and deceleration of the drawing table must be frequently and repeatedly carried out due to the intermittent movement of the drawing table.
Furthermore, in the “step & repeat” method, when any one of the optical modulation elements malfunctions, the optical modulation element concerned produces a pixel defect on the drawn circuit pattern. Accordingly, even if only one of the optical modulation elements malfunctions, it is necessary to exchange the exposure unit with a new exposure unit before a circuit pattern can be drawn without any pixel defect.